New research going deep into the heart of Mount St. Helens could eventually help to predict when it and other volcanoes will erupt.

No one can forecast exactly when the next big volcanic eruption is going to happen. But research examining a volcano's history might someday help scientists predict when the next one could blow its top.

In a new study in Science, Kate Saunders from the University of Bristol and her team used a collection of more than 300 orthopyroxene crystal samples that date to the 1980 to 1986 eruptions of Mount St. Helens. The layers or "zones" of the crystals—produced by the interaction of magma, rock, and gas inside a volcano—grow in something like concentric circles (think tree rings). Because the researchers could chemically distinguish the division between one zone and another, they were able to estimate how long it took for each zone to form. "Elements move between one zone and another to maintain equilibrium," Saunders says, "and we know how fast elements move in different crystals, so we can work out how long it took them to reach equilibrium."

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This kind of "zoning" occurs because of changes in pressure, temperature, and water content in the volcano's magma. The upshot is that, depending on the kind of elements found within each zone, the team got a sense of what forces were at play inside the volcano as the zone was made.

By studying St. Helens, for instance, Saunders and colleagues pieced together that increases in seismic activity cause these kinds of crystals to crank up their growth. When magma pulses into a chamber where crystals are already forming, the crystals develop iron-rich cores and magnesium-rich rims. On the other hand, crystals already stuck inside the magma as it enters a new chamber could cool off, causing exactly the opposite kind of zoning—magnesium-rich cores and iron-rich rims.

Today most techniques for monitoring volcanoes are focused on obvious signs, such as small earthquakes and gas releases. Researchers have analyzed crystals before, but this study is one of the first to use those chemical markers to put a time stamp on each zone within the crystal. Connecting crystal formation with a specific time frame could have implications for predicting the next big eruption.

Only not directly, says Daniel Morgan, a professor of volcanology and petrology at the University of Leeds. "It's still tricky," he says. "Obviously if you're looking at crystals from erupted volcanoes it's not going to be a predictive tool." And there's currently no safe or practical way of taking crystal samples from inside an active volcano. "But now we can start seeing things like the heartbeat or the pulse of a volcano," Morgan says.

By understanding what conditions led to a past eruption, scientists may be able to calculate what phase comes next in the volcano's life cycle. "We can use these techniques at every volcano, even if they aren't monitored now," Saunders says. "We can examine the rocks and understand what happened in the past. If it reawakens, we will have a better idea of what to anticipate."